bootmii??????

ok i was reading the softmod guid and i noticed that it says this

If ever you get bricked then copy the bootmii folder and NAND backup onto your card, do steps a) & b) again, using Preloader to get to HBC if necessary, then select the icon with the arrow going from the SD card to the chip. It is not recommended to restore your NAND if you have Bootmii installed as an IOS.

so any way what i REALY want to know is why It is not recommended to restore your NAND if you have Bootmii installed as an IOS.
if any one can tell me why then please do because i had to install bootmii as a IOS and what is the point of even installing the bootmii if It is not recommended to restore your NAND if you have Bootmii installed as an IOS.